We use Flashget (http://www.flashget.com/en/download.htm), with the FlashGot (https://addons.mozilla.org/en-US/firefox/addon/220) extension for Firefox. We use it for just that purpose - to make the most of off-peak time. Usually we have it set up to begin downloading at 2 PM and finish at 7 AM; it also shuts down the computer when you're done.